Interference pigments and devices are well known. Usually, optically-variable thin-film pigment flakes are prepared either by chemical deposition of dielectric layers onto flaked substrates or by deposition of combinations of transparent dielectric layers, semi-opaque metal layers, and metal reflecting layers onto a flexible web in vacuum to form a multilayered thin film interference structure. The structure is released later from the web after deposition in vacuum and sized in subsequent technological process. The metal-dielectric interference structure typically has at least one metal reflecting layer, at least one transparent dielectric layer and at least one semi-transparent metal layer, while all-dielectric interference structures are built from dielectric layers with different indices of refraction. Various combinations of these layers can be utilized to achieve the desired optically variable effect.
Changes of color in a pigment result from the interference of light beams reflected from thin layers of the stack. When an incident light beam encounters the first layer of a metal-dielectric-metal interference pigment, one fraction of the light is reflected back and the other fraction passes through the first layer into the second. The transmitted portion of the beam is then partially reflected by the third layer and retransmitted through the second layer. A fraction of the reflected wave passes through the first layer where it may constructively or destructively interfere with the light reflected from the surface of the filter. Maximum destructive reflective light interference occurs when the thickness of the layers are an odd number of quarter waves, whereas maximum constructive light interference occurs when the thickness of the layers are an even number of quarter waves.
The color reflected from the interference pigment depends on the path length of light passing through the dielectric material. When the pigment is observed with light at normal incidence, a certain color is seen, orange for example. When the angle of incidence and internal reflection in the interference pigment increases, the optical path length through the dielectric material becomes shorter than at normal incidence and the color reflected from the surface will change to green. At normal observation angle or angle of incident light, the curve of spectral reflectance of the pigment may have one or several peaks in the long wave range of the visible spectrum. When the angle increases the peak or peaks shift to the area of short wavelengths causing change of a reflected color from orange to green.
Color-shifting optical interference devices are used to authenticate products and documents because copies of such articles made on conventional color printers do not achieve the color-shifting effect(s). Optical interference devices are applied as foils or printed. In one example, an all-dielectric or metal-dielectric metal multilayered optical stack was applied to a substrate, such as a sheet of paper, as an optical foil. The optical stacks were made from vacuum-deposited metal films and dielectric films having low and high indices of refraction. The authenticating devices appeared red when viewed normally. As the angle of observation increased from the normal, their peak reflectance gradually shifted towards the blue part of the spectrum (“down-spectrum shift”).
Optically variable devices are also made by applying color-shifting flakes in a suitable vehicle, such as a paint vehicle or an ink vehicle, to a surface. Metal-dielectric-metal color-shifting thin film flakes and coatings have been formed by deposition of a semi-transparent metal layer upon a flexible web, followed by a dielectric layer, a metal reflecting layer, another dielectric layer, and finally another semi-transparent metal layer. The thin film layers are ordered in a symmetric fashion so that the same intended color is achieved regardless of which lateral face is directed towards the incident radiation.
All-dielectric designs have the same reflectance from either side, whether they are symmetrical or not. All-dielectric dichroic paint flakes had an optical design (L/2 H L/2)n where L and H designated a quarterwave optical thickness of the low and high refractive index materials, respectively, such that L/2 represented an eighth-wave optical thickness of low refractive index material. Reflective colors of all-dielectric flakes were unsaturated.
Further improvements in the optical characteristics of thin film flakes, which may be used in paints and inks for decorative and anti-counterfeiting applications, have been made. Symmetrical multilayer optical devices (e.g. flakes and foils) were composed either of transparent all-dielectric stacks, or transparent dielectric and semi-transparent metallic layered stacks. In the case of an all-dielectric stack, the optical coating was made of alternating layers of materials with high and low indices of refraction. Suitable materials include zinc sulfide or titanium dioxide for the high index layers, and magnesium fluoride or silicon dioxide for the low index layers. Reflectance peaks move to the short-wave region of the spectrum with increasing observation angle.
Usually, plots of spectral reflectance of the interference pigments have one or several reflectance peaks responsible for appearance of a color. If the plot has one peak, with maximum at 650 nm for example, the color reflected from the pigment will be red. If the plot has two peaks, with maxima at 650 nm and 450 nm for example, the color reflected from the pigment will be magenta in the resultant mixture of red (650 nm) and blue (450 nm) colors. Those who are skilled in the art adjust the color and color shift of the pigment by changing the optical design of the stack to place and shift the peak or peaks of spectral reflectance in the direction providing desirable color change.
However, tuning the optical design of a flake or foil has several limitations in the resultant color performance. First of all, reflectance peaks on the curve of spectral reflectance of the pigment do not shift through the entire length of visible spectrum. This means that the colors of the interference pigment do not cover all four quadrants of the color gamut with increasing observation angle. Another limitation is that the peaks can be wide or narrow. Narrow highly reflective peaks provide clear and saturated colors while wide peaks represent color blends. Presence of a second peak or even several peaks on the curve of spectral reflectance of the pigment makes the color of the pigment less saturated.
Another technique is based on multiple periods of dielectric spacer-metal absorber and constructed on a reflective metal layer. The optical design of the structures provides purposeful suppression of peaks of reflectance to produce additional chromatic effects. It was shown that with utilization of two or more periods it is possible to suppress peaks in a wave form to achieve color shifting effects other that those that can be achieved in a single period design.
To precisely describe color values of an object, it is useful to utilize the XYZ tri-stimulus color coordinate system (CIE XYZ) developed by the Commission Internationale de 1'Elclairage (CIE), currently used as a standard in the industry. In this system, colors can be related completely and accurately through the variables X, Y, and Z, which are determined mathematically as the integrals of three distribution functions covering the visible spectrum, which ranges from about 380 nm to about 770 nm, with the reflectance or transmittance curve and the energy distribution of the light source. The variables x, y, and z, which are normalized values of X, Y, and Z, respectively, are known in the art as the chromaticity coordinates, and are routinely used in the industry to quantify aspects of color such as purity, hue, and brightness.
Another standard in the industry is known as the L*a*b* color space defined by CIE. In this color space, L* indicates lightness and a* and b* are the chromaticity coordinates. With respect to the (CIE XYZ) coordinate system, the CIE L*a*b* equations are:L*=116(Y/Yw)1/3−16a*=500[(X/Xw)1/3−(Y/Yw)1/3]b*=200[(Y/Yw)1/3−(Z/Zw)1/3]Where Xw, Yw, and Zw are the X, Y, and Z values for a white reference under a specific illuminant.
In the L*a*b* chromaticity diagram, the a* axis is perpendicular to the b* axis, with increasingly positive values of a* signifying deepening chroma of red and increasingly negative values of a* signify deepening chroma of green. Along the b* axis, increasingly positive values of b* signify deepening chroma of yellow, whereas increasingly negative values of b* indicate deepening chroma of blue. The L* axis indicating lightness is perpendicular to the plane of the a* and b* axes. The L* axis along with the a* and b* axes provide for a complete description of the color attributes of an object. Hue h* of a color is an attribute of a visual perception where an area appears to be similar to one of the colors, red, yellow, green, and blue, or to a combination of adjacent pairs of these colors considered in a closed ring. The chroma C* of the color is an attribute of color used to indicate the degree of departure of the color from a gray of the same lightness. Chroma can be calculated as:C*=(a*2+b*2)1/2Chroma equals zero at the center of coordinates and increases according to the distance from the center.